ENSO impacts – rainfall

The complex interactions between the ocean, atmosphere and adjacent landmasses across the Pacific mean that ENSO events have impacts on weather in areas outside the tropical Pacific region. El Niño and La Niña events are associated with distinct climatic conditions around the Pacific.

La Niña events are associated with greater convection over the warmer ocean to Australia's north. Typically this leads to higher than average rainfall across much of Australia, particularly inland eastern and northern regions, sometimes causing floods.

During El Niño events, the ocean near Australia is cooler than usual, bringing lower than average winter–spring rainfall over eastern and northern Australia. Although most major Australian droughts have been associated with El Niño events, widespread drought is certainly not guaranteed when an El Niño is present.

Rainfall patterns during La Niña and El Niño events

Australian rainfall data for 13 of the strongest 'classic' or 'canonical' events (having the typical autumn to autumn pattern of evolution and decay) since 1900 have been combined to form a composite of average impacts of La Niña and El Niño events upon rainfall across Australia.
Each map shows mean rainfall deciles, where green to blue tones indicate above-average to very much above-average rainfall totals and yellow to red tones indicate below-average to very much below-average rainfall. Note that the rainfall patterns can vary significantly from one event to the next.

Deciles

Deciles are calculated by (1) taking all available data (say, annual Australian rainfall from 1901 to 2000), (2) ordering them from lowest to highest, and (3) dividing them into 10 separate groups of equal size. Each group is called a decile – the lowest 10 per cent of historical values is decile 1, the next lowest 10 per cent will be decile 2, and so on, up to the highest 10 per cent of historical values which lie in decile 10. This creates a scale against which we can rank the amount or intensity of a measurement or event.